Punching Shear Strength of RC Interior Slab-Column Connections Retrofitted by UHPC overlay

Abstract

The flat plate system is subjected to the combination of locally high negative bending moments and shear forces around the columns, which increases the susceptibility of this zone to brittle punching shear failure. The punching shear failure, in other words, two-way shear failure, is one of the most widely reported types of failure in buildings with flat plate system. As can be observed in many collapse accidents, it may eventually lead to a hazardous progressive collapse of the entire structure, accompanied by a truncated cone above the column. Therefore, various retrofitting strategies have been investigated and can be applied to it to improve its performance. The topping a layer of cementitous composite material is one of the retrofitting strategy genrally used in working place due to its economic efficiency and ease of construction. The insufficiency of punching shear capacity can be retrofitted by adding a thin layer of UHPC with or without steel re-bars over existing RC section. Ultra-High Performance Concrete (UHPC), developed in the mid-1990s, is a new material which has attracted many researchers and engineers for practical construction applications in structures owing to its splendid mechanical performances. Additionally, it was demonstrated by previous study that the UHPC overlay increases structural rigidity, and bending and shear strength of RC slab-column connections. However, the punching shear strength of such composite members is not defined in current design codes. The present study, thus, focusing on how the UHPC overlay contributes to the structural response of the UHPC-RC composite slab, a set of concentric loading tests was conducted to evaluate the punching shear resistance of the RC slab-column connections retrofitted by the UHPC overlay. Five specimens were constructed on half scale and designed to be failed in punching shear mode. The test variables are the UHPC overlay thickness (0mm, 30mm, and 50mm) and the amount of steel re-bars in it (0, D10@180mm, and D10@90mm). Before casting the UHPC overlay, surface of all retrofitting targets was roughened by sand-blasting method. The test results showed that by adding a thin layer of UHPC over the RC substrates, considerable increases in global punching shear resistance and structural rigidity were achieved. Furthermore, two governing failure modes based on the cracking patterns were observed: 1) diagonal shear failure in RC section (Mode 1) and 2) debonding failure at the interface (Mode 2). For verifying applicability of current design expressions, refined design expressions by reflecting the retrofitting effect in the form of effective flexural depth and effective reinforcement raio were discussed, however, were still conservative. Based on the test results, an analytical method to evaluate the structural response of the UHPC-RC composite slab was proposed based on CSCT with slight modification. To estimate each contribution of the UHPC overlay and the RC substrate to the global punching shear resistance, two free body diagrams were formulated from a slab portion. Additionally, focusing on the observed governing failure modes, two capacity curves were proposed. The predicted failure mode and maximum punching shear resistance, then, were determined by the intersection point of the demand curve and one of the capacity curves occurring at smaller slab rotation. For the validation, the experimental results were compared with the predicted results by the proposed method. The predictions show a good agreement with the test results with a small value of COV(=2.26%) and the closest average to 1.0(=1.003).